Various devices and systems have been designed to suppress fires started by rupture of fuel tanks by armor-piercing shells. These have been principally based on methods whereby a device which detects the flame automatically releases a suppressant, such as freon. Infra-red and ultra-violet optical detectors and thermocouple heat detectors have been the most common devices for this purpose, although wire grids have also been used to detect the passage of the shell. Electrically-operated and squib-operated valves have been tried as means of release of the suppressant from a more or less remote storage cylinder through the nozzle to the point of flame.
Conventional detection and triggering devices require a lapse of time after fuel tank rupture and flame start, and consequently, may allow a fire to get too much of a start or to produce hot spots which can ignite secondary fires, even though the original fire was extinguished. Extremely rapid suppression is especially important in gasoline fires. All optical and thermal flame detection and triggering devices exhibit finite even though short delay times. Further time is required for valving and travel of the suppressant from the cylinder through the nozzle to the seat of the flame. As examples of the times involved: the electronic amplifier requires about 6 milliseconds, a squib-operated valve 5 ms or less, an electronically actuated valve approximately 50 ms, and flow through the nozzle after release uses from 88 to 200 ms, depending on the distance to the flame. Thus, the over-all delay time is between 100 and 300 milliseconds. Added to this is the time for the fireball to develop to the point where the detector is activated. This latter can only be eliminated by the use of grid detector placed on the fuel tank.